In a number of circumstances, e.g., taking non-invasive in vivo measurements, the sample is inhomogeneous. Because of this inhomogeneity, errors are often introduced into the measurement process. In vivo non-invasive measurements of substances such as glucose or hemoglobin using optical systems are particularly susceptible to this problem. Measurement through a finger can have problems with variations of pathlength, variations of water content, density differences along with pathlength, shunting of signal, relative motion of sources and detectors and a host of other problems. To ameliorate these problems, a variety of techniques have been attempted. For example, in pulse oximetry, spring loaded clamps or adhesive tape are used to hold the source and detector system to the finger. This clamping eliminates some of the problems of relative motion but does not cure other sources of variance. In fact, clamping of the finger or earlobe to make non-invasive measurements can cause new problems by deformation of the tissue in an inconsistent and irreproducible way.
U.S. Pat. Nos. 5,424,545 and 5,434,412, both issued on applications of Block and Sodickson, the disclosures of which are incorporated herein by reference, are directed to different methods of attempting to cure the problems caused by inhomogeneity of the sample. In these patents, congruent sampling is used to try to minimize the difference in pathlength from the light source to the detector. These patents use optical systems which make either the light sources or detectors (or both) optically congruent; that is, the detectors view the same portion of the sample and source through the same solid angle and at the same distance. While these systems provide a benefit in reducing several of the sources of the inaccuracy caused by an inhomogeneous sample, particularly those associated with differences in pathlength, they cannot solve all of the problems. In addition, because of the need for prisms and other optical components to make the pathlengths equal, these systems are inherently bulky.
Another approach to dealing with some of the problems in output caused by the inhomogeneity of the sample is shown in the Dähne, U.S. Pat. No. 4,655,225, the disclosure of which is incorporated herein by reference. This patent uses an integrating sphere to capture light exiting a finger without regard to the directional aspect. The input port of the integrating sphere is placed next to the finger so that all of the light transmitted through the finger enters the integrating sphere and is captured so as to increase signal. A problem with this type of apparatus is that it also allows any light that is shunted (e.g., light that does not go through the area of interest) to be collected as well. In addition, because the light used to illuminate the finger may illuminate areas that have inhomogeneities, thus causing scattering, the use of the integrating sphere as shown in the Dähne patent captures light from these inhomogeneous areas as well.
U.S. Pat. No. 5,672,875, also on an application of Block and Sodickson, the disclosure of which is incorporated herein by reference, discloses another attempt to minimize the problems caused by scattering in an inhomogeneous sample. The apparatus and method shown in this patent require that the illuminating and/or detected radiation is from a limited solid angle, thereby excluding scattered radiation. This is the opposite approach to the Dähne patent, which includes all the scattered radiation as well as the non-scattered. However, even using this approach, the inhomogeneous nature of the sample may be a problem. Further, by limiting the solid angle from which radiation is used to illuminate the sample or from which the sample radiation is collected, the amount of radiation which can be detected is reduced. This is a particular problem with measurements of glucose, since it is a trace constituent rather than a major constituent of blood, and the amount of signal needed should be maximized, not minimized.
Thus, the inhomogeneities of samples cause serious problems in measurement. It is important, however, to be able to make measurements under these conditions. For example, many diabetics need to take glucose measurements numerous times during the day. Some form of continuous glucose monitor, particularly one that is not bulky, would provide significant benefits to these patients but none is presently available. Thus, a wearable device to measure glucose would be advantageous.
Accordingly, an object of the invention is to provide an apparatus and method for reducing sources of variance in measurement of an inhomogeneous sample.
Another object of the invention is to provide a method and apparatus with reduced bulk for measuring constituents of a sample such as glucose.
A further object of the invention is to provide a device which can measure concentration of optical properties of a solution or sample which is inhomogeneous.
These and other objects and features of the invention will be apparent from the following description and the drawing.